Should I unregister from event on form closing?

Question

I have following code:

private void someButton_Click(object sender, EventArgs e)
{
    SomeForm f = new SomeForm();
    this.SomeEvt += f.someFunc;
    this.AnotherEvt += f.anotherFunc;
    f.Show();
}

Should I unregister f.someFunc from this.SomeEvt and f.anotherFunc from this.AnotherEvt?

I don't want to perform neither f.anotherFunc nor someFunc when f is closed

And if I should do unregister, then how would I do that because there no longer SomeForm f after ending of this function?

I'm using .Net framework 4.0 and WinForms.

Also read from: http://stackoverflow.com/questions/4172809/should-i-unsubscribe-from-events — Habib, Jul 27 '16 at 12:36
It depends on what do you expect to have in the case when `AnotherEvt` event is *fired*, but `f` has been *closed* (i.e. do you want to perform `f.anotherFunc` when `f` is closed) — Dmitry Bychenko, Jul 27 '16 at 12:38
@DmitryBychenko, No, I dont want to perform `f.anotherFunc` when `f` is closed — Krigga, Jul 27 '16 at 12:42
Then add up `f.CloseForm += OnMyFormClose` and implement `OnMyFormClosed` where *unregister* the events: `this.SomeEvt -= f.someFunc; this.AnotherEvt -= f.anotherFunc;` — Dmitry Bychenko, Jul 27 '16 at 12:44
I've re-opened the query since the question is actually "How should I unregister if I seem to have no access to the local variable" — Dmitry Bychenko, Jul 27 '16 at 12:51
Is this WinForms? Or WPF? Or WebForms? Or Silverlight? Or ...? — Uwe Keim, Jul 27 '16 at 12:55
What happens if `SomeForm f` is closed after the form that creates it? — Martin Brown, Jul 27 '16 at 13:10

Dmitry Bychenko · Accepted Answer · 2016-07-27T12:59:09.057

4

According to your answer to my comment:

...I don't want to perform f.anotherFunc when f is closed

you should unregister the event, e.g. with lambda:

private void someButton_Click(object sender, EventArgs e)
{
    SomeForm f = new SomeForm();
    this.SomeEvt += f.someFunc;
    this.AnotherEvt += f.anotherFunc;

    f.FormClosed += (ss, ee) => {
      this.SomeEvt -= f.someFunc;
      this.AnotherEvt -= f.anotherFunc;
    };

    f.Show();
}

edited Jul 27 '16 at 12:59

answered Jul 27 '16 at 12:50

Dmitry Bychenko

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I never realised the irony in registering an event to unregister another event before seeing it written in a lambda... – Marwie Jul 27 '16 at 13:26
@Marwie: there's an irony in the very basic termine *callback function* – Dmitry Bychenko Jul 27 '16 at 13:31
Given that `this` is not passed to `f.Show()`, is there a risk of `SomeForm` being closed well after the form with the `someButton_Click` event? In this case wouldn't the reference from `SomeForm` back to `this` form cause the garbage collector to fail to collect `this` form? – Martin Brown Jul 27 '16 at 13:37
@Martin Brown: you're quite right, if `MainForm` (let name `this` like that) is closed it could not be collected by GC until `SomeForm` (`f`) has been closed. I don't think this delay being *that* critical; in case if *it is*, one can add one lambda more: `this.FormClosed += (ss, ee) => f.Close();` – Dmitry Bychenko Jul 27 '16 at 13:44

Should I unregister from event on form closing?

1 Answers1